The thermal conductivity of the expanded material is the result of four factors, i.e., gas conduction, polymer conduction, gas convention and electromagnetic radiation in the infrared range.
Gas conduction is the most important contribution but, in general, cannot be easily controlled. In most expanded materials, in fact, polyurethane included, air replaces the entrapped expanding agent with time, thus increasing the thermal conductivity of the same (see, for example, WO 91/12289).
Electromagnetic radiation can be reduced by increasing the scattering or absorption of the incoming electromagnetic waves.
Most organic materials show narrow absorption peaks and are therefore inadequate for interfering significantly with the characteristic infrared spectrum normally found in applications which, on the contrary, show a broad band. Thin layers of metals normally reflect electromagnetic radiations, whereas materials having a high refraction index, such as titanium dioxide or barium sulphate, promote the scattering of infrared radiation (see, for example, U.S. Pat. No. 5,312,678).
The use of carbon black has been known for a long time as a filler or pigment, or also as nucleating agent (see, for example, Chem. Abstr., 1987, “Carbon black containing polystyrene beads”). Carbon black exists in various forms depending on the starting materials and production process (see, for example, Kirk Othmer, Encyclopaedia of Chemical Technology, John Wiley and Sons, fourth edition, vol. 4, pages 1037 to 1074). Among the various types of carbon black, the most important are carbon black from oil combustion (“petroleum black”), carbon black from gas combustion, carbon black from acetylene, lamp carbon black, “channel black,” “thermal black” and electrically conductive carbon black.
These carbon blacks have diameters ranging from about 10 to 1,000 nm and very different specific surfaces (from 10 to 2,000 m2/g) in relation to the manufacturing process. These differences cause different blocking capacities of infrared waves, however, the results obtained by various authors are not consistent (see, for example, WO 90/06339, WO 94/13721 and WO 04/087798).
It is known that graphite can also be used as a black body effectively (as described, for example, in JP 63/183941, WO 04/022636, WO 96/34039). However, its use as attenuating agent of infrared radiation in polymeric foams is more recent.
GB 1,006,926 describes compositions containing materials, such as metals, Fe2O3 and graphite, which show a high absorption capacity of energy coming from an electromagnetic field.
GB 1,048,865 states that many fillers, particularly those of an inorganic origin, are opaque to infrared radiation. As a result, a polystyrene foam filled with those substances not only has a lower density, but also a better thermal insulation power with respect to non-filled polystyrene foams. Graphite is among the cited fillers.
JP 63-183941 is among the first to propose the use of various additives, active in blocking infrared rays in wavelengths ranging from 6 to 14 microns, thus obtaining thermally insulating thermoplastic resins capable of maintaining a low thermal conductivity permanently. Among all the additives, graphite is the preferred material.
DE 9305431U describes a method for preparing expanded molded articles having a density lower than 20 g/l and a reduced thermal conductivity. This result is achieved by incorporating an athermanous material, such as graphite and carbon black, in rigid polystyrene foam.
WO 96/34039 describes microcellular foams containing an infrared attenuating agent and a method for the use of the same. The infrared attenuating agent is coal or graphite, selected to have a good dispersion of the same in the polymeric matrix.
WO 98/51735 describes expandable polystyrene particulates containing 0.05-25% by weight of particles of synthetic or natural graphite, homogeneously distributed in the polystyrene matrix. Preferably, the graphite has an average diameter of 1 to 50 microns, an apparent density ranging from 100 to 500 g/l and a surface area ranging from 5 to 20 m2/g.
WO 00/43442 describes expandable polystyrene compositions containing up to 6% of aluminium particles. Optional infrared attenuating agents include up to 2% of Sb2S3 and also carbon black or graphite.
US 2001/036970 describes foams having a good balance between sound absorption capacity, a low thermal conductivity and generally a low water content. Active additives in infrared are graphite, titanium dioxide and all carbon blacks known in the art, such as furnace carbon black, acetylene carbon black and “thermal blacks.”
From the documentation cited, it appears evident the use of graphite and carbon black as attenuating agents of infrared radiation in foams. However, there is little evidence about the relationship between the use of these athermanous materials and their actual efficacy in blocking the infrared radiation when they are incorporated into the foams.
Both carbon blacks and graphite can contain graphite crystallites, that is regular layers having a rhombohedral or hexagonal lattice of the so-called graphene sheets. The content of crystallite phase and the coherence in the stacking of the layers is limited, in particular for carbon blacks and cokes (see, for example, “Size and shape of Crystallites and Internal Stresses in Carbon Blacks”, T. Ungara, J. Gubiczab, G. Tichyb, C. Panteac, T. W. Zerda—Composites: Part A, 2005).
Both the content of the crystallite phase and the stacking coherence can be increased under specific conditions (for example, by means of thermal treatment over 2,000° C.). However, only some types of pitches, cokes and coals can increase their graphitic degree by means of thermal processes (see, for example, “Recommended Terminology for the Description of Carbon as a Solid” from IUPAC Recommendations, 1995).
Graphite crystallites are easily broken by mechanical shear action or by means of chemical expansion of intercalate compounds. The hexagonal structure is thus subdivided into very small scales, until a substantially amorphous structure is generated, corresponding to a typical coherence length of the crystal of less than 5 nm and a stacking length of the crystal of less than 2 nm, as described in Y. Chen, M. J. Conway, J. D. Fitzgerald, J. S. Williams, L. T. Chadderton “The nucleation and Growth of Carbon Nanotubes in a Mechano-Thermal Process”, Carbon (2004) and in J. Y. Huang, “HRTEM and EELS Studies of Defect Structure and Amorphous Like Graphite Induced by Ball Milling”, Acta mater, Vol. 47, Nr. 6 (1999).
The above-mentioned crystallites have a strong interaction with the electromagnetic waves not only in the infrared spectrum. In particular, it is known, for example, from U.S. Pat. No. 4,005,183, that not succeeding in aligning the coal planes in a crystallite with respect to any other plane, prevents the material from developing typically graphitic properties, such as high thermal and electrical conductivity and electromagnetic coupling.
Useful instruments for analyzing the graphitic structure include Raman spectroscopy and X-ray diffraction, from whose analysis it is possible to compute the crystallographic parameters of the graphite crystallites and the dimensions of the same (see, for example, “Eighth Nanoforum Report Nanometrology”, July 2006, Nanoforum Org.).
It is possible to disperse many organic and inorganic compounds in graphite material, to obtain a composite (see, for example, U.S. Pat. No. 5,888,430). A more restricted group of these compounds show a molecular hindrance which is compatible with the hexagonal crystalline structure and with the inter-layer distance of graphene. In this case, an intercalate compound of graphite (GIC, i.e., Graphite Intercalate Compound) is formed and described in, for example, US 2003/0157015. These compounds can improve the compatibility of graphite in the polymeric matrix, or the thermal and electric conductivity.
The dispersion and/or intercalation of molecules inside the graphene layers may have a significant impact on the crystallite morphology (see, for example, “Improved Cycling Behaviour for Li-Doped Natural graphite Anode for Lithium Secondary Batteries”, Y. T. Lee, C. S. Yoon, S. H. Park, Y. K. Sun, Abs. 68, 204th Meeting, (2003) The Electrochemical Society).
Graphite can be incorporated into expanded polymeric matrixes in several ways. It can be added as an external additive on the polymeric beads before expansion and molding. In this way, however, the graphite material is not uniformly distributed and consequently the efficacy of the athermanous agent is reduced.
A common process consists of incorporating the graphite, together with an expanding agent, into a composition based on vinyl aromatic or urethane polymers, mixing all the additives carefully in the molten polymer, cooling and expanding, as described, for example, in JP 63-183941, GB 1,006,926 or WO 96/34039.
The beads of vinyl aromatic expandable polymers are mainly produced by means of suspension processes. These processes have been extensively described in the art. The polymerization reaction can be thermally started, through a free-radical or anionic process. Details of these processes can be found in U.S. Pat. Nos. 2,656,334 and 3,817,965.
Processes based on the suspension technology have the drawback of requiring a great quantity of water to be disposed off. Furthermore, a sieving operation is required as spherical beads having a wide diameter distribution are often obtained.
The incorporation of graphite material is a problem, as it inhibits the peroxide catalysis and makes the suspension instable. Many solutions of this problem have been proposed, as mentioned, for example, in U.S. Pat. No. 4,360,611 or in WO 98/51734 or WO 00/29471.
The uniform distribution of these pigments, especially at high content, is also a considerable problem (see, for example, WO 94/13721).
An alternative to suspension polymerization is represented by the process which consists of the mixing of a molten polymer with the expanding agent and possibly other additives (such as graphite) and the subsequent granulation of the composition thus obtained (see, for example, GB 1,062,307, U.S. Pat. No. 5,108,673, U.S. Pat. No. 5,573,790 and EP 668,139).
The product obtained from these processes is generally characterized by a cellular structure of the expanded material which is irregular and too large. The dimension of the cell and the cellular structure obtained by the expansion are of decisive importance for reaching ideal insulating properties and a good surface of the expanded material. Consequently, the addition of nucleating agents is frequently required. EP 126,459 describes a method for solving these defects by means of annealing the expandable granulates, under pressure, at a temperature higher than the glass transition temperature of the expandable polymer.
Moreover, the expandable resin leaving the cutting head is difficult to cut into granules, due to its natural tendency to expand. The incorporation of inorganic additives and, in particular, inorganic athermanous additives, makes this operation even more difficult.
WO 00/43442 states that athermanous materials have a strong nucleation effect, as a result, that underwater granulation, under pressure, is necessary to prevent expansion in the cutting chamber itself.
This method includes a particular cutting head, where the expandable resin is extruded through a large number of small holes. It is known that this method is difficult, as the surface temperature of the cutting head is very close to the temperature of the water, which, in turn, is normally close to the solidification temperature of the polymer.
Furthermore, the polymeric flow in the holes of the cutting head is at very high shear rate, as the diameter of the holes must be very limited to obtain a bead size suitable for various applications. Therefore, according to WO 00/43442, it is not possible to obtain particle size under 1 mm with this type of granulation.
Similarly, US 2005/0156344 describes the influence of the geometry of die holes (such as hole diameter, L/D ratio, inlet and outlet cone angles), the temperature of the molten product and plasticizers, on the swelling and therefore on the final diameter of the bead. It is stated that expandable resins may contain many additives. There are no examples, however, of granulates containing graphite materials.
The above-mentioned WO 98/51735 describes expandable styrene polymers containing particles of synthetic or natural graphite, homogeneously distributed in polystyrene. These compositions are obtained by mixing graphite in styrene according to an aqueous suspension process, or by mixing graphite and expanding agent in polystyrene in an extruder, with a subsequent granulation of the so-obtained composition. In the few examples relating to products manufactured according to this second procedure, the graphite content is limited to 2%.
Furthermore, as the thermal conductivity of graphite materials is typically several orders of magnitude higher than that of polymers, a polymeric foam having a high content of graphite material can show a higher overall thermal conductivity, especially at a density of the expanded material higher than about 20 g/l, if compared with a similar product but with a lower content of athermanous agent. Consequently, if, on the one hand, the use of these athermanous agents decreases the resulting conductivity in the expanded foam by reduction of the infrared transmission, on the other hand, it causes its increase by an increment in the conductivity of the solid material.
To ensure an improved thermal insulation, it is therefore fundamental not only to control the concentration of the athermanous material, but also its localization in the polymeric matrix. To our present knowledge, so far no effective solution has been proposed for solving these problems, in the group of relevant products.
It could therefore be helpful to provide granulates of expandable composite materials based on vinyl aromatic polymers, to which an agent has been added to improve the thermal insulation, which, after further transformation, allow low-density, expanded articles to be prepared having a thermal conductivity sufficiently low to be used for obtaining enhanced thermal insulation properties.
It could also be helpful to provide granulates of expandable composite materials from which, after further transformation, expanded panels can be obtained, having a high thermal insulation performance, to satisfy national standards, with a minimum thickness of the panel and at a cost compatible with commercially available products.
It could further be helpful to provide expandable composite materials which, in their final form of expanded article, after expansion and molding, satisfy the self-extinguishing specifications of the B2 test, according to the regulation DIN 4102 part 2, with a reduced use of self-extinguishing additives.
It could yet further be helpful to provide expandable composite materials which, after expansion and molding, allow expanded articles to be obtained which do not present undesired worsening in mechanical properties.
It could still further be helpful to provide a process for the production of granules of expandable composite materials mentioned above, as well as the expanded articles obtained from the granules after expansion and molding, having a high content of closed cells and a homogeneous cellular dimension, in the range from 60 to 400 microns.